It is normal for the PATH variable to be there already. It is an important part of the operating system but it is common to make changes to it. As it is an important function of your computer you should

If by "comprehensive" you mean does it come with all of the SDK as well as the IDE? Then yes. That said, I think I remember it only coming with the most current SDK version pre-installed. If you want earlier versions of the SDK (to target older devices), then you have to open the SDK Manager and download the versions that you are interested in. Depending on how many you actually want, the download will take a good bit of time--so prepare to wait.

This download is 510MB and includes the following:
The Eclipse Integrated Development Environment (IDE) + ADT plugin
Android SDK Tools
Android Platform-tools
The latest Android platform
The latest Android system image for the emulator

Once you have the ADT Bundle installed you should be ready to start following the training website tutorials.

Note: there is also and Android Studio IDE which is still in 'Early Access' (i.e. not finished) and is a bit different from the Eclipse IDE. The training website I've linked above uses the Eclipse IDE so it's best to use this.

I'm curious if you've any background in programming. It would make things a bit easier if you do.

One requirement that I forgot you need is the Java Development Kit (JDK).
I've included a bit of information here about Java but you can skip to the end if you just want to download a JDK.

The JDK is different from the Java Runtime Environment (JRE) that most computers have installed. Specifically the JDK includes a Java compiler (javac.exe) as well as the full JRE. The Android Software Development Kit (SDK) is essentially extra Java classes that can be compiled in your Java application which means your Java Application is now an Android Application i.e. in programming for Android you're actually programming in Java.

So you need to download a version of the JDK. Firstly, there are different Editions of the JDKs. These are Standard Edition (SE), Enterprise Edition (EE) and Micro Edition (ME). These are the most used but you'll only need SE to build your Android Apps. The difference between them is basically what classes are available to you when developing your application.

There are 2 versions that are currently available. Version 7 and version 8. Version 7 is probably the safer bet at this point as Java 8 is very new and there might be some incompatibility between the Android SDK and Java 8.

Scroll down to the Java SE 7u55 section and choose the download button with JDK written over it. You can ignore the other bits and pieces for now.

Once you've installed the JDK there's one more step to do before you're ready to go with it (I'm assuming you're using Windows here).
You need to add the "bin" directory within the Java install directory to your "PATH" environment variable.

Open Control Panel and the clcik the "System" icon

Click the "Advance System Settings" link from the left hand menu (Windows 7 and 8)

In the dialog box that opens click the "Environment Variables" button

Highlight the "PATH" entry in the bottom list and click "Edit..."

Add a Semi-Colon followed by the location of the Java 'bin' folder i.e. ;C:\Program Files (x86)\Java\jdk1.7.0_55\bin\

Click OK on each of the windows.

To confirm you have the JDK installed and available, open a command prompt and type in:
javac -version

You should get back a line that says: javac 1.7.0_55

Once you confirm this is done you should be ready to ready to go with the Tutorials.

You might also want to check that you have all the recommended options installed for the Android SDK. Have a look at the following link which tells you about the Android SDK Manager (You can open this using the Eclipse IDE through the "Window" toolbar menu).http://developer.android.com/sdk/installing/adding-packages.html

It is normal for the PATH variable to be there already. It is an important part of the operating system but it is common to make changes to it. As it is an important function of your computer you should take a few minutes to learn about.

The PATH tells the computer where to look for executable files.

It is best to use the Command Prompt to understand how the path works.
When you open the Command Prompt it will normally open up so that you are in your own user folder i.e. it will show C:\Users\myUser>
If you created a program (myProg.exe) and stored it in your user folder you could open the Command Prompt and run it by simply typing in myProg.exe
But, if you opened the Command prompt and changed directory to the root of the C: drive (cd \) then typing in myProg.exe would give and error.
This is because myProg.exe doesn't exist in the root of the C: drive.
To run myProg.exe when you are in a different folder you would need to type in the full path of the executable i.e. C:\users\myUser\myProg.exe
The program would be found as you've told your computer exactly where it is. When you do it in this way you are using "Absolute Addressing". This means you are specifying exactly where the file is starting at the root of the disk.

So this is where the PATH variable comes into it. When you try to run a program the PATH gives the computer a specific list of folders to search in for that program.
Lets take the example where your PATH is just "C:\windows\system32".
If you open the command prompt you will be in "C:\Users\myUser". Now if you type in "ipconfig.exe" the computer will look in the folder you are in (C:\Users\myUser) and it will also look in the folder specified in your PATH variable (C:\Windows\system32). Because the "ipconfig.exe" program exists in "C:\Windows\System32" the computer will find it and run the program even though you do not have that program open in the Command Prompt.
On the other hand, if you typed in "notepad.exe" you would get an error. This is because "notepad.exe" exists in "C:\Windows" which is not in your PATH.

If you were to edit you PATH to include the C:\Windows folder then typing in "notepad.exe" would open Notepad no matter what directory you are in.
To add a folder to your PATH you edit the variable to include the folder and make sure that there is a semi-colon (;) separating the folders i.e. "C:\windows\system32;C:\windows" or "C:\windows;C:\windows\system32"

.....

So coming back to your issue. Hopefully you understand what you need to do now.
To be able to run the java.exe or javac.exe programs from any folder you need to add the folder they exist in to your PATH variable. The Eclipse IDE will use the PATH to find the java programs too.
You edit your existing PATH.
If you try to create a new PATH variable you will overwrite the existing PATH which will cause a lot of issues for you.
You need to add C:\Program Files (x86)\Java\jdk1.7.0_55\bin\ to the start or end of the PATH. Remember to put a semi-colon between this folder and the next one in the PATH.

Once you've updated your PATH open a new Command Prompt (You need to open a new one after making this change to see the result).
Type in: javac -version
Check that you get the right version number back.

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